Defense Secretary Robert Gates said in March that the Iranians were playing a "double game" inside Afghanistan by striving for good relations with Kabul while undermining the U.S. effort. Weeks later, Adm. Mike Mullen, chairman of the Joint Chiefs of Staff, said there was evidence that Iran was smuggling weapons into Afghanistan.

The U.S. military has not made public evidence supporting its suspicions, but some analysts say that if the allegations are true, that means Iran and the Taliban are willing to work with a traditional religious Muslim rival to get the Americans.

"If the Taliban is getting support from Iran, they know that it's not out of some love for them," said Mohsen Milani, an Iran scholar at the University of South Florida. "It's only because they are being used as useful idiots to do the dirty work that Iran doesn't want to do itself or that Iran is not capable of doing."